What is AO in the context of volumetric rendering?
Basically, when rendering a surface, you are asking “Does this surface point see the environment sphere or not?”
But in the case of volumetric rendering, you would have to ask the question about every point in the volume, taking into account all other particles. You cannot do this with Magma because currently you cannot shoot rays against particles, and you cannot check against particles of the same cloud. You could of course use a clone of the point cloud, but there is no good way to look up along a ray to figure out the occlusion.
Something we have done in the past is placing multiple lights on the vertices of a Geosphere around the scene and calculating the lighting using Krakatoa. Krakatoa shadows result in a kind of occlusion test because each particle inside the volume receives less light based on how many particles are between it and the light source. The more lights you use, the better but slower the calculations become.
We are considering some grid-based approach for future versions of Krakatoa to calculate AO and GI, but there is really nothing there to do it right now. It might be doable with Stoke MX 2.0, but I have not tried.